JP2011174151A - Cold-rolled steel sheet for high-tension pipe excellent in surface quality and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cold-rolled steel sheet for high-tension pipe excellent in surface quality, and a method for manufacturing the same. <P>SOLUTION: The cold-rolled steel sheet has a composition comprising, by mass, 0.050-0.070% C, 0.8-1.5% Si, 1.8-2.5% Mn, ≤0.020% P, ≤0.010% S, 0.010-0.100% Sol.Al, ≤0.005% N and the balance being Fe and unavoidable impurities. The structure thereof is a two-phase structure comprising a ferrite phase and a martensite phase. The cold-rolled steel sheet is obtained by: starting rapid cooling of a slab within 1 sec after hot-rolling so as to achieve a temperature drop of >100°C but <300°C at an average cooling rate of >10°C/s; winding the same at a winding temperature of 500-600°C; subjecting the same to cold-rolling; and continuously annealing the same in a furnace atmosphere with a hydrogen concentration of ≥3 vol.% and a dew point of ≤-30°C. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a cold-rolled steel sheet for high-strength pipes excellent in surface quality suitable mainly as a pipe material and a method for producing the same.

  In recent years, improving the fuel efficiency of automobiles has become an important issue from the viewpoint of preventing global warming. Along with this, there is an active movement to reduce the thickness of the vehicle body by increasing the strength of the vehicle body material and to reduce the weight of the vehicle body itself. However, the reduction in ductility is caused by the reduction in thickness and strength of the steel plate as the body material. For this reason, the present situation is that development of a material having both high strength and high ductility is desired.

In response to such demands, up to now, the design has been changed to high alloying by adding steel components such as Si and Mn, and the ductility reduction has been compensated by improving the material properties of the material itself.
However, when the steel plate after such a design change is used as a material for high-tensile pipe use, there are the following problems. For example, when a steel plate is used for the headrest component, the steel plate is first formed into a pipe, and then the surface is subjected to a glossy and beautiful treatment such as chrome plating. However, in the case of high-alloyed steel sheets with the addition of Si, Mn, etc., there are cases where defects such as poor gloss occur after plating due to the oxides of Si and Mn having a strong affinity for oxygen. It became so.
As steels having both high strength and high ductility, Patent Documents 1 to 3 disclose high-strength hot-rolled steel sheets having good ductility. However, none of these patent documents considers the cold-rolled steel sheet (cold-rolling material for pipes), and the C content of the steel sheet is 0.15 mass% or more (or more than 0.15 mass%). It has a high component design, and there is a problem that its practical performance is low, such as a decrease in spot weldability and hole expandability.

Japanese Patent Publication No. 6-41617 Japanese Patent Publication No. 5-65566 JP-B-5-67682

  In view of such circumstances, an object of the present invention is to provide a cold-rolled steel sheet for a high-tensile pipe having excellent surface quality and a method for producing the same.

In order to solve the above-described problems and to produce a cold-rolled steel sheet for high-tensile pipes having excellent surface quality, the present inventors have conducted intensive research from the viewpoint of steel sheet production conditions. As a result, the following knowledge was obtained.
It has been found that the production conditions of hot rolling and continuous annealing have a great influence on the surface beauty after plating, which is the final product.
By defining the steel composition, hot rolling and continuous annealing production conditions, the state of oxides (Si oxides, etc.) present on the steel sheet surface changes and it is possible to suppress the generation of oxides. . As a result, it was found that the surface quality of the final product was improved.
Specifically, when hot rolling is performed on a steel slab adjusted to a predetermined composition, cooling is started within 1 second after hot rolling (after finish rolling) as a cooling condition after finish rolling, and 10 ° C Rapid cooling at an average cooling rate of more than 100 ° C and a temperature drop of more than 100 ° C and less than 300 ° C, and winding at a winding temperature of 500 to 600 ° C suppresses the generation of oxides without impairing surface quality. Steel plate can be obtained.
In continuous annealing after cold rolling, among the annealing conditions, the atmosphere in the furnace is set to a hydrogen concentration of 3 vol% or more, and the dew point is set to -30 ° C or less, thereby suppressing the formation of surface oxides and achieving a good final The surface quality of the product can be obtained.

This invention is made | formed based on the above knowledge, The summary is as follows.
[1] Component composition is mass%, C: 0.050-0.070%, Si: 0.8-1.5%, Mn: 1.8-2.5%, P: 0.020% or less, S: 0.010% or less, Sol.Al: 0.010- High-strength pipe with excellent surface quality characterized by containing 0.100%, N: 0.005% or less, the balance being iron and inevitable impurities, and having a two-phase structure of ferrite phase and martensite phase Cold rolled steel sheet.
[2] After the slab having the composition described in [1] above is hot-rolled, cooling is started within 1 second, and the temperature drop is more than 100 ° C and less than 300 ° C at an average cooling rate of more than 10 ° C / s. After cooling at a coiling temperature of 500 to 600 ° C., cold rolling, and then continuously annealing at a dew point of −30 ° C. or lower in a furnace atmosphere with a hydrogen concentration of 3 vol% or higher. A method for producing a cold-rolled steel sheet for high-tensile pipes with excellent surface quality.

  In addition, in this specification,% which shows the component of steel is mass% altogether. In the present invention, the “cold-rolled steel sheet for high-strength pipe” is a cold-rolled steel sheet having a tensile strength TS of 680 MPa or more (preferably 680 to 850 MPa).

  ADVANTAGE OF THE INVENTION According to this invention, the cold-rolled steel plate for high tension pipes excellent in surface quality is obtained. By using the cold-rolled steel sheet for high-strength pipes of the present invention as a material for high-tensile pipes, for example, it has excellent surface quality, and can have both high strength and high ductility, greatly contributing to high performance. There is an excellent effect.

  Details of the present invention will be described below.

1) Component composition
C: 0.050 ~ 0.070%
C is an important element for strengthening the martensite of the quenched structure. If the C content is less than 0.050%, the effect of increasing the strength is insufficient. On the other hand, if the amount of C exceeds 0.070%, the welded portion is broken in the cross tension test in spot welding, so the joint strength is significantly reduced. For the above reasons, the C content is 0.050% or more and 0.070% or less.

Si: 0.8-1.5%
Si is effective for enhancing the ductility of steel having a two-phase structure of ferrite and martensite. If the Si content is less than 0.8%, the effect is not sufficient. On the other hand, if it exceeds 1.5%, a large amount of Si oxide is formed on the surface of the steel sheet, resulting in poor quality when plated for final use. From the above, the Si content is 0.8% to 1.5%.

Mn: 1.8-2.5%
Mn is an important element for suppressing the formation of ferrite in the annealing zone in a continuous annealing furnace. If it is less than 1.8%, the effect is not sufficient. On the other hand, if it exceeds 2.5%, slab cracking occurs in the continuous casting process. Based on the above, the Mn content is 1.8% to 2.5%.

P: 0.020% or less
P is desirably reduced in the steelmaking process as much as possible in order to deteriorate spot weldability. If it exceeds 0.020%, the spot weldability deteriorates remarkably, so the P content needs to be 0.020% or less.

S: 0.010% or less
It is desirable to reduce S in the steel making process as much as possible in order to deteriorate spot weldability and bending workability. If it exceeds 0.010%, the spot weldability and bending workability will deteriorate significantly, so the S content must be 0.010% or less.

Sol.Al:0.010~0.100%
Al is added to deoxidize and precipitate N as AlN. If Sol.Al is less than 0.010%, the effect of deoxidation and denitrification is not sufficient. On the other hand, if it exceeds 0.100%, the effect of adding Al becomes saturated and uneconomical. Accordingly, the amount of Sol.Al is set to 0.010% or more and 0.100% or less.

N: 0.005% or less
Since N deteriorates the formability of the material steel plate, it is desirable to remove and reduce it as much as possible in the steelmaking process. However, if N is reduced more than necessary, the refining cost increases, so 0.005% or less is made virtually harmless.

  The balance is Fe and inevitable impurities.

2) Microstructure The cold-rolled steel sheet for high-strength pipes of the present invention has a two-phase structure of a ferrite phase and a martensite phase. At this time, the ferrite phase is preferably 60% to 65% by volume, and the martensite phase is preferably 35% to 40% by volume. By setting it as such a structure | tissue, it has the characteristic excellent in the balance of ductility and hole expansibility.
Ferrite phase volume fraction of 60% to 65% (preferred range)
If the volume fraction of the ferrite phase is less than 60%, the ductility may decrease, so 60% or more is preferable. On the other hand, if the volume fraction of the ferrite phase exceeds 65%, the tensile strength may not be ensured. Therefore, the volume fraction of the ferrite phase is preferably 60% or more and 65% or less.
The volume ratio of martensite phase is 35% to 40% (preferable range)
The martensite phase works effectively to increase the strength of steel. However, if the volume ratio exceeds 40%, the ductility may decrease. Further, if the volume ratio is less than 35%, the strength and hole expansibility may be deteriorated. Therefore, the volume ratio of the martensite phase is preferably 35% or more and 40% or less.

  In addition, since the objective of this invention can be achieved if the structure of the said microstructure is satisfied, as phases other than a ferrite and a martensite, a bainite phase and an austenite phase are each less than 2% (volume ratio), respectively, or further Inevitable cementite phase, AlN and MnS can be included, and these structures are harmless to the properties (surface quality and the like) in the cold-rolled steel sheet of the present invention.

  The volume ratio of the ferrite phase, martensite phase, bainite phase, and austenite phase in the cold-rolled steel sheet of the present invention means the ratio of the area of each phase (area ratio) to the observed area. For example, after polishing the plate thickness cross section parallel to the rolling direction of the steel sheet, the area ratio is corroded with 3% nital, observed with 10 fields of view at a magnification of 2000 using a SEM (scanning electron microscope), and commercially available image processing It can be obtained using software.

3) Manufacturing conditions The cold-rolled steel sheet for high-strength pipes of the present invention starts cooling within 1 second after hot-rolling a slab having the above-mentioned composition, and is 100 ° C. at an average cooling rate exceeding 10 ° C./s. Rapid cooling at a temperature drop of less than 300 ° C, winding at a coiling temperature of 500 to 600 ° C, cold rolling, and then -30 ° C in an atmosphere in a furnace with a hydrogen concentration of 3 vol% or more Continuous annealing at the following dew points.
Details will be described below.

Steel adjusted to the above component composition is melted in a converter or the like, and is slabed by a continuous casting method or the like, and is hot-rolled.
The steel slab to be used is preferably produced by a continuous casting method in order to prevent macro segregation of components, but may be produced by an ingot casting method or a thin slab casting method. After manufacturing the steel slab, in addition to the conventional method of cooling to room temperature and then heating again, without cooling to room temperature, insert it into a heating furnace as it is, or carry out slight heat retention Energy saving processes such as direct feed rolling and direct rolling, which are rolled immediately, can be applied without any problem.

Slab heating temperature: 1100 ℃ or higher (preferred conditions)
As for the slab heating temperature, low-temperature heating is preferable in terms of energy, but if the heating temperature is less than 1100 ° C, the carbide cannot be sufficiently dissolved, or the risk of trouble occurring during hot rolling due to an increase in rolling load increases. Problems arise. Therefore, the slab heating temperature is preferably 1100 ° C. or higher. The slab heating temperature is preferably set to 1300 ° C. or lower because of an increase in scale loss accompanying an increase in oxidation.
From the viewpoint of preventing troubles during hot rolling even if the slab heating temperature is lowered, a so-called sheet bar heater that heats the sheet bar may be used.

Finishing rolling finish temperature: A ₃ points or more (preferred conditions)
If it is less than finish rolling temperature A ₃ points, during rolling alpha (ferrite) and gamma (austenite) are generated, the band-like structure is liable to generate in the steel plate, or causing anisotropy in material properties, It may cause a decrease in workability. Therefore, the finish rolling end temperature is preferably set to A ₃ transformation point or more.

Next, it starts cooling within 1 second after hot rolling, rapidly cools at an average cooling rate of more than 10 ° C / s with a temperature drop of more than 100 ° C and less than 300 ° C, and a coiling temperature of 500 to 600 ° C. Take up with.
Cooling starts within 1 second after hot rolling, and when the average cooling rate exceeds 10 ° C / s and the rapid cooling average cooling rate is 10 ° C / s or less with a temperature drop of more than 100 ° C and less than 300 ° C, The surface quality deteriorates due to an increase in the oxide layer. Therefore, the average cooling rate is over 10 ° C / s. On the other hand, when the average cooling rate exceeds 100 ° C./s, the material may be hardened. From the above, the average cooling rate is more than 10 ° C./s, preferably more than 10 ° C./s and not more than 100 ° C./s.
In addition, when cooling is started for more than 1 second after hot rolling, an oxide layer on the surface of the steel sheet is increased, and the surface properties are deteriorated.
Further, when the cooling temperature width (temperature drop amount) of the average cooling rate exceeding 10 ° C./s is 300 ° C. or more, the material becomes hard due to rapid cooling. In addition, when the cooling temperature width (temperature drop amount) is 100 ° C. or less, an oxide layer on the surface of the steel sheet is increased and the surface properties are deteriorated.

Winding temperature: 500-600 ° C
If the coiling temperature is less than 500 ° C., the steel sheet becomes hard and cold rolling becomes difficult. On the other hand, when the coiling temperature exceeds 600 ° C., an oxide layer on the surface of the steel sheet is increased and the surface properties are deteriorated.
By performing cooling and winding under the above conditions after hot rolling, it is possible to obtain a steel sheet in which the generation of oxide is suppressed on the steel sheet surface without damaging the material after cold rolling and annealing.

Next, the obtained hot-rolled steel sheet is pickled as necessary, and then cold-rolled.
In pickling, it is preferable to remove the scale on the surface of the steel sheet with hydrochloric acid. Cold rolling is not particularly limited, and can be performed by a commonly performed method. However, from the viewpoint of improving ductility and hole expandability due to refinement of the structure, the rolling reduction of cold rolling is preferably 50 to 85%.

Next, continuous annealing is performed at a dew point of −30 ° C. or lower in a furnace atmosphere having a hydrogen concentration of 3 vol% or higher.
When continuous annealing is performed in an atmosphere in a furnace with a hydrogen concentration of less than 3 vol%, the reducing ability is inferior, and the formation of an oxide layer on the steel sheet surface cannot be sufficiently suppressed. Moreover, when continuous annealing is performed at a dew point exceeding −30 ° C., oxidation of the steel sheet surface is promoted, and the surface properties of the steel sheet deteriorate. In order to obtain the above microstructure, the annealing and cooling conditions are 750 to 850 ° C, hold for 10 seconds or more, and then gradually cool to 550 to 650 ° C, then average cooling exceeding 10 ° C / s It is preferable to rapidly cool to 50 ° C. or less at a speed and further to perform tempering at 150 to 300 ° C. By controlling the annealing conditions in such a range, the above microstructure can be obtained stably.

By performing continuous annealing under the above conditions, it is possible to suppress the formation of oxides that hinder the surface properties of the steel sheet, and to obtain a good surface quality of the final product.
As described above, the cold-rolled steel sheet for high-tensile pipes with good surface quality according to the present invention is manufactured.
The cold-rolled steel sheet may be subjected to temper rolling for adjustment of shape correction, surface roughness, and the like. Moreover, there is no inconvenience even if various plating treatments such as galvanization and treatments such as resin or oil coating and various coatings are performed.

(Example 1)
Steel having the composition shown in Table 1 and the balance being Fe and inevitable impurities was melted in a converter and cast to produce a slab. The slab was heated at a heating temperature of 1250 ° C. and then hot-rolled to a thickness of 3.2 mm. The finishing rolling temperature was 870 ° C. Subsequently, cooling was started within 1 second, rapid cooling was performed at an average cooling rate of 100 ° C./s with a temperature drop of 200 ° C., and winding was performed at a winding temperature of 560 ° C. Next, pickling and cold rolling were performed to obtain a plate thickness of 1.5 mm, and further continuous annealing was performed. The continuous annealing was heated to 830 ° C., then cooled to 600 ° C. at an average cooling rate of about 10 ° C./s, and rapidly cooled to water temperature in jet water having a water temperature of 20 ° C. Further, after tempering at a temperature of 200 ° C. and cooling to room temperature, 0.3% (elongation rate) temper rolling was performed.

The cold-rolled steel sheet obtained as described above was examined for cross-sectional microstructure, tensile properties, hole expansibility, and surface quality. The results obtained are shown in Table 2.
In addition, the cross-sectional microstructure of the steel sheet is revealed with a 3% nital solution (3% nitric acid + ethanol), and the depth direction plate thickness 1/4 position is observed 10 times with a scanning electron microscope at a magnification of 2000 times, Image analysis processing was performed using the photographed tissue photograph, and the fraction of the ferrite phase was quantified (in addition, commercially available image processing software can be used for the image analysis processing).
The area ratio of the martensite phase was quantified with image processing software by taking SEM photographs at an appropriate magnification of 1000 to 3000 times depending on the fineness of the structure. The area ratios of the ferrite phase and martensite phase thus obtained were evaluated as the volume ratio of each phase.
For mechanical properties, a tensile test based on JISZ2241 was conducted using a JIS5 test piece sampled so that the tensile direction was perpendicular to the rolling direction of the steel sheet. YP (yield point), TS (tensile strength) EL (elongation) was measured. TS is 680MPa or more, and EL is 15% or more.
Further, λ (hole expansion rate) was measured by performing a hole expansion test according to JFST1001. λ is better than 60%.
As for the surface quality, after the strip plate was subjected to a bending test, Ni plating treatment (adhesion amount of 10 to 15 μm) was applied, and the case where there was no gloss failure visually was judged as good (◯), and the case where there was a gloss failure was judged as poor (×).

From Table 2, the inventive examples were excellent in all of mechanical properties, hole expansibility and surface quality.
On the other hand, in the comparative example of steel number 2, the mechanical properties were good, but the surface quality was inferior. In the comparative example of steel number 3, the tensile strength and hole expansibility were inferior because of the low Mn content. In the comparative example of Steel No. 4, since the C amount was high, the elongation and hole expansibility were inferior. In the comparative example of Steel No. 5, since the amount of Mn was high, elongation, hole expansibility, and surface quality were inferior. In the comparative example of steel number 6, since Si was low, the hole expandability was inferior.

(Example 2)
Cold rolled steel sheets were produced using the steel numbers 1 and 2 in Table 1 used in Example 1 under the hot rolling and continuous annealing conditions shown in Table 3. The slab heating temperature was 1250 ° C., the finished thickness of hot rolling was 3.2 mm, the thickness after cold rolling was 1.5 mm, and the temper rolling elongation was 0.3%. The resulting cold-rolled steel sheet was examined for cross-sectional microstructure, tensile properties, hole expansibility and surface quality. The investigation of the cross-sectional microstructure, tensile properties, hole expansibility and surface quality is the same as in Example 1. The results obtained are shown in Table 3 together with the production conditions.

From Table 3, the code F, which is an example of the present invention, obtained good results in tensile properties, hole expansibility, and surface quality.
On the other hand, all other comparative examples resulted in poor surface quality. In the comparative examples of reference signs A and B, the cooling ability after hot rolling was low, so the oxidation of the steel sheet surface was promoted and the surface quality was inferior. In the comparative example of C, since the cooling ability after hot rolling was high, the material was hardened and it was difficult to perform cold rolling. In the comparative example of reference D, the coiling temperature after hot rolling was high, so that the oxidation of the steel sheet surface was promoted and the surface quality was inferior. In the comparative example of E, since the coiling temperature after hot rolling was low, the material was cured and cold rolling became difficult. In the comparative example indicated by G, since the dew point in the furnace during continuous annealing was high, the reduction of the steel sheet surface was inferior and the surface quality was inferior. In the comparative example of the code | symbol H, since the hydrogen concentration in the furnace atmosphere at the time of continuous annealing was low, the reduction | restoration of the steel plate surface was inferior and surface quality was inferior. Since the amount of Si was high, the surface quality of the steel sheet was deteriorated.

Claims (2)

  Component composition is mass%, C: 0.050-0.070%, Si: 0.8-1.5%, Mn: 1.8-2.5%, P: 0.020% or less, S: 0.010% or less, Sol.Al: 0.010-0.100%, N: 0.005% or less, the balance is made of iron and inevitable impurities, and the structure has a two-phase structure of a ferrite phase and a martensite phase. steel sheet.   Cooling is started within 1 second after hot rolling the slab having the component composition according to claim 1 and rapidly cooled at an average cooling rate of more than 10 ° C / s with a temperature drop of more than 100 ° C and less than 300 ° C. Then, after rolling at a coiling temperature of 500 to 600 ° C., cold rolling, and then continuously annealing at a dew point of −30 ° C. or less in an atmosphere in a furnace having a hydrogen concentration of 3 vol% or more A method of manufacturing cold-rolled steel sheets for high-strength pipes with excellent quality.